Sample Modes

The PXIe-4309 supports averaging and single-sample acquisition sampling modes.

The sampling mode is selected based on the timing mode.

In this section convert is a single ADC quantized digitization of the input signal, and sample is the data returned from hardware.

Averaging Acquisition

The PXIe-4309 supports averaging in hardware. Based on the sample rate, the PXIe-4309 averages as many converts per sample as possible to deliver the best resolution for a given sample rate. Refer to the PXIe-4309 Specifications for Noise versus Sample Rate information.

The following figure shows an averaging acquisition of two samples of a single channel.

Figure 15. Single Channel Averaging

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Note Samples are timestamped to the time when the sample clock takes place but are acquired throughout the sample period. This introduces a phase shift of half a sample period when channel expanding to other types of data acquisition devices.
Tip An NI-DAQmx multidevice task, or channel expansion, expands the channels of a DAQ device to include the channels of another DAQ device. You can use channel expansion with to include multiple devices in a single task, let NI-DAQmx automatically synchronize them, and enable trigger skew correction for the devices. By having NI-DAQmx enable trigger skew correction, the driver can compensate for the skew of a trigger signal across multiple devices by locking the trigger to the reference clock and allows the devices to respond to a trigger on a subsequent edge of that clock rather than immediately.

The following figure shows an averaging acquisition of two converts per sample of two channels on the same ADC. The settling time increases as the aggregate sample rate decreases to allow for more settling time to reduce settling and ghosting errors.

Figure 16. Multiple Channel Averaging

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The following table shows the max sampling rate based on the maximum number of channels per ADC.
Table 7. Maximum Sample Rate
Maximum Channels per ADC Maximum Sample Rate Aggregate Rate
1 2 MHz 2 MHz
2 200 kHz 400 kHz
3 133 kHz
4 100 kHz

Single Convert Acquisition

The PXIe-4309 takes a single convert per channel and returns that as a sample. The following figure shows a single convert acquisition of a single channel.

Figure 17. Single Convert Acquisition of a Single Channel

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The following figure shows a single convert acquisition of two channels on the same ADC. The settling time increases as the aggregate sample rate decreases to allow for more settling time to reduce settling and ghosting errors.

Figure 18. Single Convert Acquisition of Two Channels on the Same ADC

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Table 8. Maximum Sample Rate (Hardware-Timed Single Point, On-Demand, and External Sample Clock)
Maximum Channels per ADC Maximum Sample Rate Aggregate Rate
1 900 kHz 900 kHz
2 180 kHz 360 kHz
3 120 kHz
4 90 kHz